Strange behaviour while I'm texturing an object using the projection painting technique
Image data is treated separate to 3d model data, you must save the image data as a separate action from saving blender's data.

If an image is saved as an external file, then you need to choose Image->Save Image in the UV/Image Editor. If this is the first time you have saved the image you also want to save the blend file so that you also save the location of the image.

If an image is to be saved as a packed image within a blend file, you need to choose Image->Pack Image, then save the blend file so the image is saved to disk with the rest of the blend file.

If you alter a packed image, you need to use Image->Pack As PNG to update the packed data, then save the blend file to save it to disk.

The Image->Pack As PNG menu option is only available when a packed image has been changed.